Systems relating to the authentication of objects such as documents, passports, credit cards and currency often use an anti-counterfeiting means in the form of indicia that are difficult to identify or copy in the hopes of deterring a would-be counterfeiter from making unauthorized copies. For example, decorative or identifiable distinctive marks on currency offer limited protection. However, as more sophisticated anti-counterfeiting means are used, it is usually only a matter of time until even these means are successfully but illegitimately copied.
Currently, optically-variable security devices, such as thin films, holograms, gratings, and micro-prisms, are commonly used to protect valuable documents and credit cards; their optical features can be easily recognized by the public at large. Thin film optically-variable devices are described in a paper entitled Optical Interference Coatings for Inhibiting of Counterfeiting by J. A. Dobrowolski et at. in Optica Acta, 1973, Vol. 20, No. 12, 925-937. Optically-variable thin film security devices exhibiting a color change with a shift in viewing angle have also been disclosed in U.S. Pat. No. 3,858,977 issued in January of 1975 in the names of Baird et al. Such devices are used on large denominations of Canadian bank notes and on drivers' licenses in some jurisdictions, however, the efficacy of these devices is questionable. In an attempt to offer additional security protection U.S. Pat. No. 5,009,486 issued Apr. 23, 1991 in the names of Dobrowolski et al. describes a form depicting, optical interference authenticating device. Dobrowolski et al. teach the introduction of additional patterns in thin film security devices by using masks during their deposition or by laser ablation to remove one or more layers of the thin film system.
Since these and many other security devices depend largely on verification by visual inspection, a person performing the verification must be aware of what features to look for and must possess the visual acuity to perform the task; this is not always the case. As counterfeiters become more successful in their attempts to copy security devices, it is believed that in the future there will be a need for additional security.